The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Multiple Input Multiple Output (MIMO) systems generally leverage aspects of intersymbol interference to potentially increase the bandwidth efficiency of existing spectra. In wireless communication, radio waves may not propagate in a straight line between a transmitter and a receiver, e.g., radio waves can bounce and scatter randomly off objects in the environment. This scattering, known as multipath, results in multiple copies of a transmitted signal arriving at a receiver via different scatter paths. MIMO leverages multipath to enhance transmission accuracy and allow multiple signals to be broadcast at the same frequency. This is done by treating the multiple scatter paths as separate parallel sub channels, each capable of bearing distinct data.
MIMO operates by splitting a discrete outbound signal into multiple parallel substreams using an array of transmitter antennas to simultaneously launch (or transmit) the parallel substreams. Another array of antennas in the receiving device is used to pick up (or receive) the multiple transmitted spatial streams and their scattered signals. Each receive antenna picks up all of the incident transmitted spatial streams superimposed as observed components of the received signal vector, not separately. However, the multiple spatial streams are all scattered slightly differently, since the multiple spatial streams originate from different transmit antennas located at different points in space. These scattering differences allow the spatial streams to be identified and recovered from the observed components of the received signal vector.
Beamforming significantly improves the performance of MIMO by spatially separating the transmitted data streams. With transmit beamforming, weights are applied to the signal transmitted by each transmit antenna to “steer” the composite transmission toward the intended receiver. The weights for each transmit antenna for each spatial stream, typically expressed as a steering matrix, are derived from channel state information (CSI) for each spatial stream. The CSI describes the channel travelled by a given spatial stream between the transmit antenna array and the receiver. The CSI is used to derive the weights for the signal from each transmitter antenna that will cause the composite signal to travel back along the channel to the intended receiver.
Orthogonal Frequency Division Multiplexing (OFDM) divides a communications channel into a number of equally spaced frequency bands, called tones. A subcarrier carrying a portion of the user information is transmitted in each tone. In OFDM, each tone is orthogonal (independent) to every other tone. OFDM spread spectrum distributes the data over a large number of tones that are spaced apart at precise frequencies. In OFDM, the original data is split into parallel streams and each stream is mapped to a different tone. The tones are combined using the inverse fast Fourier transform (IFFT) to yield a time-domain waveform packet to be transmitted. In a multiple-access system, groups of tones can be assigned to different users. Multiple users can thus share the same overall bandwidth. The typical frequency channel for legacy wireless communication systems is 20 MHz. IEEE 802.11n specifies a 40 Mhz channel, having 20 MHz upper and lower bands. Other systems can employ channels having other bandwidths, e.g., 80 MHz channel, 120 MHz channel, 160 MHz channel, and so on.
In the case of multiple-access MIMO-OFDM, different weights may be applied to selected tones in the transmitted packet so the tones are steered along a channel between the transmitter and a wireless device assigned to the tones. Meanwhile other tones in the packet may be weighted differently so that the tones are steered to the wireless devices to which the tones are assigned. MIMO in combination with OFDM constitutes the basis for many wireless communication standards, such as IEEE802.11n.